Sound waves are pressure waves that travel through air, gas, solids, and liquids,
but cannot travel through space or a vacuum unlike radio (electromagnetic) waves.
Pressure waves can have frequencies up to about 50 kHz. Sound or sonic waves
start at 16 Hz and go up to 20 kHz; above 30 kHz sound waves are ultrasonic. Sound
waves travel through air at about 340 m/s (depends on temperature, pressure, and
the like.). The amplitude or loudness of sound is measured in phons.
Sound pressure levels (SPL) are units often used in the measurement of sound
levels and are defined as the difference in pressure between the maximum pressure
at a point and the average pressure at that point. The units of pressure
are normally expressed as follows: